ADT(abstract data type) in C
On 2017/07/30 at 21:30
When tracing the code of libnetfilter_conntrack libray, I found the use of abstract data type implementation in C is very interesting and elegant. Its OOP behavior is the same as the following C++ code:
class nf_conntrack {
public:
/* some public method...*/
void set_attr(const enum nf_conntrack_attr type, void* value);
void* get_attr(const enum nf_conntrack_attr type);
int compare(const nf_conntrack *other);
/* blah blah... */
private:
struct __nfct_tuple repl;
struct __nfct_tuple master;
uint32_t timeout;
uint32_t mark;
/* some other private field */
};
It restrict users from using nf_conntrack's private field and only allow them using its public member function. The library did it by forward declaring the nf_conntrack and its public function in libnetfilter_conntrack/libnetfiter_conntrack.h and hide the private internal members in internal/internal.h and internal/object.h
libnetfilter_conntrack/libnetfiter_conntrack.h:
/* forward declaring nf_conntrack */
struct nf_conntrack;
extern struct nf_conntrack *nfct_new(void);
extern void nfct_destroy(struct nf_conntrack *ct);
/* setter */
extern void nfct_set_attr(struct nf_conntrack *ct,
const enum nf_conntrack_attr type,
const void *value);
/* getter */
extern const void *nfct_get_attr(const struct nf_conntrack *ct,
const enum nf_conntrack_attr type);
internal/object.h:
/* real nf_conntrack definition */
struct nf_conntrack {
struct nfct_tuple_head head;
struct __nfct_tuple repl;
struct __nfct_tuple master;
uint32_t timeout;
uint32_t mark;
uint32_t secmark;
uint32_t status;
uint32_t use;
uint32_t id;
uint16_t zone;
/* blah blah... */
};
In conntrack/api.c, the api implementation include internal/internal.h directly. However for users using this library, they can only use the public methods exposed by libnetfilter_conntrack.h.